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Tuesday, November 04, 2014

Verizon users might want to start looking for another provider. In an effort to better serve advertisers, Verizon Wireless has been silently modifying its users' web traffic on its network to inject a cookie-like tracker. This tracker, included in an HTTP header called X-UIDH, is sent to every unencrypted website a Verizon customer visits from a mobile device. It allows third-party advertisers and websites to assemble a deep, permanent profile of visitors' web browsing habits without their consent.

Verizon apparently created this mechanism to expand their advertising programs, but it has privacy implications far beyond those programs. Indeed, while we're concerned about Verizon's own use of the header, we're even more worried about what it allows others to find out about Verizon users. The X-UIDH header effectively reinvents the cookie, but does so in a way that is shockingly insecure and dangerous to your privacy. Worse still, Verizon doesn't let users turn off this "feature." In fact, it functions even if you use a private browsing mode or clear your cookies. You can test whether the header is injected in your traffic by visiting lessonslearned.org/sniff or amibeingtracked.com over a cell data connection.

How X-UIDH Works, and Why It's a Problem

To compound the problem, the header also affects more than just web browsers. Mobile apps that send HTTP requests will also have the header inserted. This means that users' behavior in apps can be correlated with their behavior on the web, which would be difficult or impossible without the header. Verizon describes this as a key benefit of using their system. But Verizon bypasses the 'Limit Ad Tracking' settings in iOS and Android that are specifically intended to limit abuse of unique identifiers by mobile apps.

Because the header is injected at the network level, Verizon can add it to anyone using their towers, even those who aren't Verizon customers.

We're also concerned that Verizon's failure to permit its users to opt out of X-UIDH may be a violation of the federal law that requires phone companies to maintain the confidentiality of their customers' data. Only two months ago, the wireline sector of Verizon's business was hit with a $7.4 million fine by the Federal Communications Commission after it was caught using its "customers' personal information for thousands of marketing campaigns without even giving them the choice to opt out." With this header, it looks like Verizon lets its customers opt out of the marketing side of the program, but not from the disclosure of their browsing habits.